The molecular architecture of lipid membranes - New insights from hydration-tuning infrared linear dichroism spectroscopy

A new, precise method of adjustment and variation of the hydration degree of materials, is presented. This humidity-titration technique has proved to be practical, relatively fast to achieve, accurate, and, last but not least, well-defined in a thermodynamic sense. It is used in combination with infrared (IR) attenuated total reflection (ATR) spectroscopy to study the hydration of selected phospholipids on a molecular level. Examples of hydration-driven, lyotropic phase transitions are presented and discussed. The second part of the article deals with the relationship between polarised IR absorption spectroscopy and the degree of molecular order in macroscopically oriented samples. The basic formulae for the analysis of IR dichroism data in terms of molecular order parameters are given. Aspects of the molecular order of lipid membranes, such as the orientation and conformation of phosphocholine (PC) headgroups before and after interaction with ions (Ca2+ and Mg2+), the segmential order of the acyl chains in fluid and solid phases, complex formation between water and the PC headgroups and the ordering of lipid-bound water are presented in the third part of this publication. These examples illustrate the effect of the hydration degree on the molecular architecture of lipid assemblies.